llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

//===- InstCombineShifts.cpp ----------------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the visitShl, visitLShr, and visitAShr functions.
//
//===----------------------------------------------------------------------===//

#include "InstCombineInternal.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/PatternMatch.h"
using namespace llvm;
using namespace PatternMatch;

#define DEBUG_TYPE "instcombine"

Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
  Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
  assert(Op0->getType() == Op1->getType());

  // See if we can fold away this shift.
  if (SimplifyDemandedInstructionBits(I))
    return &I;

  // Try to fold constant and into select arguments.
  if (isa<Constant>(Op0))
    if (SelectInst *SI = dyn_cast<SelectInst>(Op1))
      if (Instruction *R = FoldOpIntoSelect(I, SI))
        return R;

  if (Constant *CUI = dyn_cast<Constant>(Op1))
    if (Instruction *Res = FoldShiftByConstant(Op0, CUI, I))
      return Res;

  // (C1 shift (A add C2)) -> (C1 shift C2) shift A)
  // iff A and C2 are both positive.
  Value *A;
  Constant *C;
  if (match(Op0, m_Constant()) && match(Op1, m_Add(m_Value(A), m_Constant(C))))
    if (isKnownNonNegative(A, DL) && isKnownNonNegative(C, DL))
      return BinaryOperator::Create(
          I.getOpcode(), Builder->CreateBinOp(I.getOpcode(), Op0, C), A);

  // X shift (A srem B) -> X shift (A and B-1) iff B is a power of 2.
  // Because shifts by negative values (which could occur if A were negative)
  // are undefined.
  const APInt *B;
  if (Op1->hasOneUse() && match(Op1, m_SRem(m_Value(A), m_Power2(B)))) {
    // FIXME: Should this get moved into SimplifyDemandedBits by saying we don't
    // demand the sign bit (and many others) here??
    Value *Rem = Builder->CreateAnd(A, ConstantInt::get(I.getType(), *B-1),
                                    Op1->getName());
    I.setOperand(1, Rem);
    return &I;
  }

  return nullptr;
}

/// Return true if we can simplify two logical (either left or right) shifts
/// that have constant shift amounts.
static bool canEvaluateShiftedShift(unsigned FirstShiftAmt,
                                    bool IsFirstShiftLeft,
                                    Instruction *SecondShift, InstCombiner &IC,
                                    Instruction *CxtI) {
  assert(SecondShift->isLogicalShift() && "Unexpected instruction type");

  // We need constant shifts.
  auto *SecondShiftConst = dyn_cast<ConstantInt>(SecondShift->getOperand(1));
  if (!SecondShiftConst)
    return false;

  unsigned SecondShiftAmt = SecondShiftConst->getZExtValue();
  bool IsSecondShiftLeft = SecondShift->getOpcode() == Instruction::Shl;

  // We can always fold  shl(c1) +  shl(c2) ->  shl(c1+c2).
  // We can always fold lshr(c1) + lshr(c2) -> lshr(c1+c2).
  if (IsFirstShiftLeft == IsSecondShiftLeft)
    return true;

  // We can always fold lshr(c) +  shl(c) -> and(c2).
  // We can always fold  shl(c) + lshr(c) -> and(c2).
  if (FirstShiftAmt == SecondShiftAmt)
    return true;

  unsigned TypeWidth = SecondShift->getType()->getScalarSizeInBits();

  // If the 2nd shift is bigger than the 1st, we can fold:
  //   lshr(c1) +  shl(c2) ->  shl(c3) + and(c4) or
  //   shl(c1)  + lshr(c2) -> lshr(c3) + and(c4),
  // but it isn't profitable unless we know the and'd out bits are already zero.
  // Also check that the 2nd shift is valid (less than the type width) or we'll
  // crash trying to produce the bit mask for the 'and'.
  if (SecondShiftAmt > FirstShiftAmt && SecondShiftAmt < TypeWidth) {
    unsigned MaskShift = IsSecondShiftLeft ? TypeWidth - SecondShiftAmt
                                           : SecondShiftAmt - FirstShiftAmt;
    APInt Mask = APInt::getLowBitsSet(TypeWidth, FirstShiftAmt) << MaskShift;
    if (IC.MaskedValueIsZero(SecondShift->getOperand(0), Mask, 0, CxtI))
      return true;
  }

  return false;
}

/// See if we can compute the specified value, but shifted
/// logically to the left or right by some number of bits.  This should return
/// true if the expression can be computed for the same cost as the current
/// expression tree.  This is used to eliminate extraneous shifting from things
/// like:
///      %C = shl i128 %A, 64
///      %D = shl i128 %B, 96
///      %E = or i128 %C, %D
///      %F = lshr i128 %E, 64
/// where the client will ask if E can be computed shifted right by 64-bits.  If
/// this succeeds, the GetShiftedValue function will be called to produce the
/// value.
static bool CanEvaluateShifted(Value *V, unsigned NumBits, bool IsLeftShift,
                               InstCombiner &IC, Instruction *CxtI) {
  // We can always evaluate constants shifted.
  if (isa<Constant>(V))
    return true;

  Instruction *I = dyn_cast<Instruction>(V);
  if (!I) return false;

  // If this is the opposite shift, we can directly reuse the input of the shift
  // if the needed bits are already zero in the input.  This allows us to reuse
  // the value which means that we don't care if the shift has multiple uses.
  //  TODO:  Handle opposite shift by exact value.
  ConstantInt *CI = nullptr;
  if ((IsLeftShift && match(I, m_LShr(m_Value(), m_ConstantInt(CI)))) ||
      (!IsLeftShift && match(I, m_Shl(m_Value(), m_ConstantInt(CI))))) {
    if (CI->getZExtValue() == NumBits) {
      // TODO: Check that the input bits are already zero with MaskedValueIsZero
#if 0
      // If this is a truncate of a logical shr, we can truncate it to a smaller
      // lshr iff we know that the bits we would otherwise be shifting in are
      // already zeros.
      uint32_t OrigBitWidth = OrigTy->getScalarSizeInBits();
      uint32_t BitWidth = Ty->getScalarSizeInBits();
      if (MaskedValueIsZero(I->getOperand(0),
            APInt::getHighBitsSet(OrigBitWidth, OrigBitWidth-BitWidth)) &&
          CI->getLimitedValue(BitWidth) < BitWidth) {
        return CanEvaluateTruncated(I->getOperand(0), Ty);
      }
#endif

    }
  }

  // We can't mutate something that has multiple uses: doing so would
  // require duplicating the instruction in general, which isn't profitable.
  if (!I->hasOneUse()) return false;

  switch (I->getOpcode()) {
  default: return false;
  case Instruction::And:
  case Instruction::Or:
  case Instruction::Xor:
    // Bitwise operators can all arbitrarily be arbitrarily evaluated shifted.
    return CanEvaluateShifted(I->getOperand(0), NumBits, IsLeftShift, IC, I) &&
           CanEvaluateShifted(I->getOperand(1), NumBits, IsLeftShift, IC, I);

  case Instruction::Shl:
  case Instruction::LShr:
    return canEvaluateShiftedShift(NumBits, IsLeftShift, I, IC, CxtI);

  case Instruction::Select: {
    SelectInst *SI = cast<SelectInst>(I);
    Value *TrueVal = SI->getTrueValue();
    Value *FalseVal = SI->getFalseValue();
    return CanEvaluateShifted(TrueVal, NumBits, IsLeftShift, IC, SI) &&
           CanEvaluateShifted(FalseVal, NumBits, IsLeftShift, IC, SI);
  }
  case Instruction::PHI: {
    // We can change a phi if we can change all operands.  Note that we never
    // get into trouble with cyclic PHIs here because we only consider
    // instructions with a single use.
    PHINode *PN = cast<PHINode>(I);
    for (Value *IncValue : PN->incoming_values())
      if (!CanEvaluateShifted(IncValue, NumBits, IsLeftShift, IC, PN))
        return false;
    return true;
  }
  }
}

/// When CanEvaluateShifted returned true for an expression,
/// this value inserts the new computation that produces the shifted value.
static Value *GetShiftedValue(Value *V, unsigned NumBits, bool isLeftShift,
                              InstCombiner &IC, const DataLayout &DL) {
  // We can always evaluate constants shifted.
  if (Constant *C = dyn_cast<Constant>(V)) {
    if (isLeftShift)
      V = IC.Builder->CreateShl(C, NumBits);
    else
      V = IC.Builder->CreateLShr(C, NumBits);
    // If we got a constantexpr back, try to simplify it with TD info.
    if (auto *C = dyn_cast<Constant>(V))
      if (auto *FoldedC =
              ConstantFoldConstant(C, DL, &IC.getTargetLibraryInfo()))
        V = FoldedC;
    return V;
  }

  Instruction *I = cast<Instruction>(V);
  IC.Worklist.Add(I);

  switch (I->getOpcode()) {
  default: llvm_unreachable("Inconsistency with CanEvaluateShifted");
  case Instruction::And:
  case Instruction::Or:
  case Instruction::Xor:
    // Bitwise operators can all arbitrarily be arbitrarily evaluated shifted.
    I->setOperand(
        0, GetShiftedValue(I->getOperand(0), NumBits, isLeftShift, IC, DL));
    I->setOperand(
        1, GetShiftedValue(I->getOperand(1), NumBits, isLeftShift, IC, DL));
    return I;

  case Instruction::Shl: {
    BinaryOperator *BO = cast<BinaryOperator>(I);
    unsigned TypeWidth = BO->getType()->getScalarSizeInBits();

    // We only accept shifts-by-a-constant in CanEvaluateShifted.
    ConstantInt *CI = cast<ConstantInt>(BO->getOperand(1));

    // We can always fold shl(c1)+shl(c2) -> shl(c1+c2).
    if (isLeftShift) {
      // If this is oversized composite shift, then unsigned shifts get 0.
      unsigned NewShAmt = NumBits+CI->getZExtValue();
      if (NewShAmt >= TypeWidth)
        return Constant::getNullValue(I->getType());

      BO->setOperand(1, ConstantInt::get(BO->getType(), NewShAmt));
      BO->setHasNoUnsignedWrap(false);
      BO->setHasNoSignedWrap(false);
      return I;
    }

    // We turn shl(c)+lshr(c) -> and(c2) if the input doesn't already have
    // zeros.
    if (CI->getValue() == NumBits) {
      APInt Mask(APInt::getLowBitsSet(TypeWidth, TypeWidth - NumBits));
      V = IC.Builder->CreateAnd(BO->getOperand(0),
                                ConstantInt::get(BO->getContext(), Mask));
      if (Instruction *VI = dyn_cast<Instruction>(V)) {
        VI->moveBefore(BO);
        VI->takeName(BO);
      }
      return V;
    }

    // We turn shl(c1)+shr(c2) -> shl(c3)+and(c4), but only when we know that
    // the and won't be needed.
    assert(CI->getZExtValue() > NumBits);
    BO->setOperand(1, ConstantInt::get(BO->getType(),
                                       CI->getZExtValue() - NumBits));
    BO->setHasNoUnsignedWrap(false);
    BO->setHasNoSignedWrap(false);
    return BO;
  }
  // FIXME: This is almost identical to the SHL case. Refactor both cases into
  // a helper function.
  case Instruction::LShr: {
    BinaryOperator *BO = cast<BinaryOperator>(I);
    unsigned TypeWidth = BO->getType()->getScalarSizeInBits();
    // We only accept shifts-by-a-constant in CanEvaluateShifted.
    ConstantInt *CI = cast<ConstantInt>(BO->getOperand(1));

    // We can always fold lshr(c1)+lshr(c2) -> lshr(c1+c2).
    if (!isLeftShift) {
      // If this is oversized composite shift, then unsigned shifts get 0.
      unsigned NewShAmt = NumBits+CI->getZExtValue();
      if (NewShAmt >= TypeWidth)
        return Constant::getNullValue(BO->getType());

      BO->setOperand(1, ConstantInt::get(BO->getType(), NewShAmt));
      BO->setIsExact(false);
      return I;
    }

    // We turn lshr(c)+shl(c) -> and(c2) if the input doesn't already have
    // zeros.
    if (CI->getValue() == NumBits) {
      APInt Mask(APInt::getHighBitsSet(TypeWidth, TypeWidth - NumBits));
      V = IC.Builder->CreateAnd(I->getOperand(0),
                                ConstantInt::get(BO->getContext(), Mask));
      if (Instruction *VI = dyn_cast<Instruction>(V)) {
        VI->moveBefore(I);
        VI->takeName(I);
      }
      return V;
    }

    // We turn lshr(c1)+shl(c2) -> lshr(c3)+and(c4), but only when we know that
    // the and won't be needed.
    assert(CI->getZExtValue() > NumBits);
    BO->setOperand(1, ConstantInt::get(BO->getType(),
                                       CI->getZExtValue() - NumBits));
    BO->setIsExact(false);
    return BO;
  }

  case Instruction::Select:
    I->setOperand(
        1, GetShiftedValue(I->getOperand(1), NumBits, isLeftShift, IC, DL));
    I->setOperand(
        2, GetShiftedValue(I->getOperand(2), NumBits, isLeftShift, IC, DL));
    return I;
  case Instruction::PHI: {
    // We can change a phi if we can change all operands.  Note that we never
    // get into trouble with cyclic PHIs here because we only consider
    // instructions with a single use.
    PHINode *PN = cast<PHINode>(I);
    for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
      PN->setIncomingValue(i, GetShiftedValue(PN->getIncomingValue(i), NumBits,
                                              isLeftShift, IC, DL));
    return PN;
  }
  }
}

/// Try to fold (X << C1) << C2, where the shifts are some combination of
/// shl/ashr/lshr.
static Instruction *
foldShiftByConstOfShiftByConst(BinaryOperator &I, ConstantInt *COp1,
                               InstCombiner::BuilderTy *Builder) {
  Value *Op0 = I.getOperand(0);
  uint32_t TypeBits = Op0->getType()->getScalarSizeInBits();

  // Find out if this is a shift of a shift by a constant.
  BinaryOperator *ShiftOp = dyn_cast<BinaryOperator>(Op0);
  if (ShiftOp && !ShiftOp->isShift())
    ShiftOp = nullptr;

  if (ShiftOp && isa<ConstantInt>(ShiftOp->getOperand(1))) {

    // This is a constant shift of a constant shift. Be careful about hiding
    // shl instructions behind bit masks. They are used to represent multiplies
    // by a constant, and it is important that simple arithmetic expressions
    // are still recognizable by scalar evolution.
    //
    // The transforms applied to shl are very similar to the transforms applied
    // to mul by constant. We can be more aggressive about optimizing right
    // shifts.
    //
    // Combinations of right and left shifts will still be optimized in
    // DAGCombine where scalar evolution no longer applies.

    ConstantInt *ShiftAmt1C = cast<ConstantInt>(ShiftOp->getOperand(1));
    uint32_t ShiftAmt1 = ShiftAmt1C->getLimitedValue(TypeBits);
    uint32_t ShiftAmt2 = COp1->getLimitedValue(TypeBits);
    assert(ShiftAmt2 != 0 && "Should have been simplified earlier");
    if (ShiftAmt1 == 0)
      return nullptr; // Will be simplified in the future.
    Value *X = ShiftOp->getOperand(0);

    IntegerType *Ty = cast<IntegerType>(I.getType());

    // Check for (X << c1) << c2  and  (X >> c1) >> c2
    if (I.getOpcode() == ShiftOp->getOpcode()) {
      uint32_t AmtSum = ShiftAmt1 + ShiftAmt2; // Fold into one big shift.
      // If this is an oversized composite shift, then unsigned shifts become
      // zero (handled in InstSimplify) and ashr saturates.
      if (AmtSum >= TypeBits) {
        if (I.getOpcode() != Instruction::AShr)
          return nullptr;
        AmtSum = TypeBits - 1; // Saturate to 31 for i32 ashr.
      }

      return BinaryOperator::Create(I.getOpcode(), X,
                                    ConstantInt::get(Ty, AmtSum));
    }

    if (ShiftAmt1 == ShiftAmt2) {
      // If we have ((X << C) >>u C), turn this into X & (-1 >>u C).
      if (I.getOpcode() == Instruction::LShr &&
          ShiftOp->getOpcode() == Instruction::Shl) {
        APInt Mask(APInt::getLowBitsSet(TypeBits, TypeBits - ShiftAmt1));
        return BinaryOperator::CreateAnd(
            X, ConstantInt::get(I.getContext(), Mask));
      }
    } else if (ShiftAmt1 < ShiftAmt2) {
      uint32_t ShiftDiff = ShiftAmt2 - ShiftAmt1;

      // (X >>?,exact C1) << C2 --> X << (C2-C1)
      // The inexact version is deferred to DAGCombine so we don't hide shl
      // behind a bit mask.
      if (I.getOpcode() == Instruction::Shl &&
          ShiftOp->getOpcode() != Instruction::Shl && ShiftOp->isExact()) {
        assert(ShiftOp->getOpcode() == Instruction::LShr ||
               ShiftOp->getOpcode() == Instruction::AShr);
        ConstantInt *ShiftDiffCst = ConstantInt::get(Ty, ShiftDiff);
        BinaryOperator *NewShl =
            BinaryOperator::Create(Instruction::Shl, X, ShiftDiffCst);
        NewShl->setHasNoUnsignedWrap(I.hasNoUnsignedWrap());
        NewShl->setHasNoSignedWrap(I.hasNoSignedWrap());
        return NewShl;
      }

      // (X << C1) >>u C2  --> X >>u (C2-C1) & (-1 >> C2)
      if (I.getOpcode() == Instruction::LShr &&
          ShiftOp->getOpcode() == Instruction::Shl) {
        ConstantInt *ShiftDiffCst = ConstantInt::get(Ty, ShiftDiff);
        // (X <<nuw C1) >>u C2 --> X >>u (C2-C1)
        if (ShiftOp->hasNoUnsignedWrap()) {
          BinaryOperator *NewLShr =
              BinaryOperator::Create(Instruction::LShr, X, ShiftDiffCst);
          NewLShr->setIsExact(I.isExact());
          return NewLShr;
        }
        Value *Shift = Builder->CreateLShr(X, ShiftDiffCst);

        APInt Mask(APInt::getLowBitsSet(TypeBits, TypeBits - ShiftAmt2));
        return BinaryOperator::CreateAnd(
            Shift, ConstantInt::get(I.getContext(), Mask));
      }

      // We can't handle (X << C1) >>s C2, it shifts arbitrary bits in. However,
      // we can handle (X <<nsw C1) >>s C2 since it only shifts in sign bits.
      if (I.getOpcode() == Instruction::AShr &&
          ShiftOp->getOpcode() == Instruction::Shl) {
        if (ShiftOp->hasNoSignedWrap()) {
          // (X <<nsw C1) >>s C2 --> X >>s (C2-C1)
          ConstantInt *ShiftDiffCst = ConstantInt::get(Ty, ShiftDiff);
          BinaryOperator *NewAShr =
              BinaryOperator::Create(Instruction::AShr, X, ShiftDiffCst);
          NewAShr->setIsExact(I.isExact());
          return NewAShr;
        }
      }
    } else {
      assert(ShiftAmt2 < ShiftAmt1);
      uint32_t ShiftDiff = ShiftAmt1 - ShiftAmt2;

      // (X >>?exact C1) << C2 --> X >>?exact (C1-C2)
      // The inexact version is deferred to DAGCombine so we don't hide shl
      // behind a bit mask.
      if (I.getOpcode() == Instruction::Shl &&
          ShiftOp->getOpcode() != Instruction::Shl && ShiftOp->isExact()) {
        ConstantInt *ShiftDiffCst = ConstantInt::get(Ty, ShiftDiff);
        BinaryOperator *NewShr =
            BinaryOperator::Create(ShiftOp->getOpcode(), X, ShiftDiffCst);
        NewShr->setIsExact(true);
        return NewShr;
      }

      // (X << C1) >>u C2  --> X << (C1-C2) & (-1 >> C2)
      if (I.getOpcode() == Instruction::LShr &&
          ShiftOp->getOpcode() == Instruction::Shl) {
        ConstantInt *ShiftDiffCst = ConstantInt::get(Ty, ShiftDiff);
        if (ShiftOp->hasNoUnsignedWrap()) {
          // (X <<nuw C1) >>u C2 --> X <<nuw (C1-C2)
          BinaryOperator *NewShl =
              BinaryOperator::Create(Instruction::Shl, X, ShiftDiffCst);
          NewShl->setHasNoUnsignedWrap(true);
          return NewShl;
        }
        Value *Shift = Builder->CreateShl(X, ShiftDiffCst);

        APInt Mask(APInt::getLowBitsSet(TypeBits, TypeBits - ShiftAmt2));
        return BinaryOperator::CreateAnd(
            Shift, ConstantInt::get(I.getContext(), Mask));
      }

      // We can't handle (X << C1) >>s C2, it shifts arbitrary bits in. However,
      // we can handle (X <<nsw C1) >>s C2 since it only shifts in sign bits.
      if (I.getOpcode() == Instruction::AShr &&
          ShiftOp->getOpcode() == Instruction::Shl) {
        if (ShiftOp->hasNoSignedWrap()) {
          // (X <<nsw C1) >>s C2 --> X <<nsw (C1-C2)
          ConstantInt *ShiftDiffCst = ConstantInt::get(Ty, ShiftDiff);
          BinaryOperator *NewShl =
              BinaryOperator::Create(Instruction::Shl, X, ShiftDiffCst);
          NewShl->setHasNoSignedWrap(true);
          return NewShl;
        }
      }
    }
  }

  return nullptr;
}

Instruction *InstCombiner::FoldShiftByConstant(Value *Op0, Constant *Op1,
                                               BinaryOperator &I) {
  bool isLeftShift = I.getOpcode() == Instruction::Shl;

  ConstantInt *COp1 = nullptr;
  if (ConstantDataVector *CV = dyn_cast<ConstantDataVector>(Op1))
    COp1 = dyn_cast_or_null<ConstantInt>(CV->getSplatValue());
  else if (ConstantVector *CV = dyn_cast<ConstantVector>(Op1))
    COp1 = dyn_cast_or_null<ConstantInt>(CV->getSplatValue());
  else
    COp1 = dyn_cast<ConstantInt>(Op1);

  if (!COp1)
    return nullptr;

  // See if we can propagate this shift into the input, this covers the trivial
  // cast of lshr(shl(x,c1),c2) as well as other more complex cases.
  if (I.getOpcode() != Instruction::AShr &&
      CanEvaluateShifted(Op0, COp1->getZExtValue(), isLeftShift, *this, &I)) {
    DEBUG(dbgs() << "ICE: GetShiftedValue propagating shift through expression"
              " to eliminate shift:\n  IN: " << *Op0 << "\n  SH: " << I <<"\n");

    return replaceInstUsesWith(
        I, GetShiftedValue(Op0, COp1->getZExtValue(), isLeftShift, *this, DL));
  }

  // See if we can simplify any instructions used by the instruction whose sole
  // purpose is to compute bits we don't care about.
  uint32_t TypeBits = Op0->getType()->getScalarSizeInBits();

  assert(!COp1->uge(TypeBits) &&
         "Shift over the type width should have been removed already");

  // ((X*C1) << C2) == (X * (C1 << C2))
  if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Op0))
    if (BO->getOpcode() == Instruction::Mul && isLeftShift)
      if (Constant *BOOp = dyn_cast<Constant>(BO->getOperand(1)))
        return BinaryOperator::CreateMul(BO->getOperand(0),
                                         ConstantExpr::getShl(BOOp, Op1));

  if (Instruction *FoldedShift = foldOpWithConstantIntoOperand(I))
    return FoldedShift;

  // Fold shift2(trunc(shift1(x,c1)), c2) -> trunc(shift2(shift1(x,c1),c2))
  if (TruncInst *TI = dyn_cast<TruncInst>(Op0)) {
    Instruction *TrOp = dyn_cast<Instruction>(TI->getOperand(0));
    // If 'shift2' is an ashr, we would have to get the sign bit into a funny
    // place.  Don't try to do this transformation in this case.  Also, we
    // require that the input operand is a shift-by-constant so that we have
    // confidence that the shifts will get folded together.  We could do this
    // xform in more cases, but it is unlikely to be profitable.
    if (TrOp && I.isLogicalShift() && TrOp->isShift() &&
        isa<ConstantInt>(TrOp->getOperand(1))) {
      // Okay, we'll do this xform.  Make the shift of shift.
      Constant *ShAmt = ConstantExpr::getZExt(COp1, TrOp->getType());
      // (shift2 (shift1 & 0x00FF), c2)
      Value *NSh = Builder->CreateBinOp(I.getOpcode(), TrOp, ShAmt,I.getName());

      // For logical shifts, the truncation has the effect of making the high
      // part of the register be zeros.  Emulate this by inserting an AND to
      // clear the top bits as needed.  This 'and' will usually be zapped by
      // other xforms later if dead.
      unsigned SrcSize = TrOp->getType()->getScalarSizeInBits();
      unsigned DstSize = TI->getType()->getScalarSizeInBits();
      APInt MaskV(APInt::getLowBitsSet(SrcSize, DstSize));

      // The mask we constructed says what the trunc would do if occurring
      // between the shifts.  We want to know the effect *after* the second
      // shift.  We know that it is a logical shift by a constant, so adjust the
      // mask as appropriate.
      if (I.getOpcode() == Instruction::Shl)
        MaskV <<= COp1->getZExtValue();
      else {
        assert(I.getOpcode() == Instruction::LShr && "Unknown logical shift");
        MaskV = MaskV.lshr(COp1->getZExtValue());
      }

      // shift1 & 0x00FF
      Value *And = Builder->CreateAnd(NSh,
                                      ConstantInt::get(I.getContext(), MaskV),
                                      TI->getName());

      // Return the value truncated to the interesting size.
      return new TruncInst(And, I.getType());
    }
  }

  if (Op0->hasOneUse()) {
    if (BinaryOperator *Op0BO = dyn_cast<BinaryOperator>(Op0)) {
      // Turn ((X >> C) + Y) << C  ->  (X + (Y << C)) & (~0 << C)
      Value *V1, *V2;
      ConstantInt *CC;
      switch (Op0BO->getOpcode()) {
      default: break;
      case Instruction::Add:
      case Instruction::And:
      case Instruction::Or:
      case Instruction::Xor: {
        // These operators commute.
        // Turn (Y + (X >> C)) << C  ->  (X + (Y << C)) & (~0 << C)
        if (isLeftShift && Op0BO->getOperand(1)->hasOneUse() &&
            match(Op0BO->getOperand(1), m_Shr(m_Value(V1),
                  m_Specific(Op1)))) {
          Value *YS =         // (Y << C)
            Builder->CreateShl(Op0BO->getOperand(0), Op1, Op0BO->getName());
          // (X + (Y << C))
          Value *X = Builder->CreateBinOp(Op0BO->getOpcode(), YS, V1,
                                          Op0BO->getOperand(1)->getName());
          uint32_t Op1Val = COp1->getLimitedValue(TypeBits);

          APInt Bits = APInt::getHighBitsSet(TypeBits, TypeBits - Op1Val);
          Constant *Mask = ConstantInt::get(I.getContext(), Bits);
          if (VectorType *VT = dyn_cast<VectorType>(X->getType()))
            Mask = ConstantVector::getSplat(VT->getNumElements(), Mask);
          return BinaryOperator::CreateAnd(X, Mask);
        }

        // Turn (Y + ((X >> C) & CC)) << C  ->  ((X & (CC << C)) + (Y << C))
        Value *Op0BOOp1 = Op0BO->getOperand(1);
        if (isLeftShift && Op0BOOp1->hasOneUse() &&
            match(Op0BOOp1,
                  m_And(m_OneUse(m_Shr(m_Value(V1), m_Specific(Op1))),
                        m_ConstantInt(CC)))) {
          Value *YS =   // (Y << C)
            Builder->CreateShl(Op0BO->getOperand(0), Op1,
                                         Op0BO->getName());
          // X & (CC << C)
          Value *XM = Builder->CreateAnd(V1, ConstantExpr::getShl(CC, Op1),
                                         V1->getName()+".mask");
          return BinaryOperator::Create(Op0BO->getOpcode(), YS, XM);
        }
        LLVM_FALLTHROUGH;
      }

      case Instruction::Sub: {
        // Turn ((X >> C) + Y) << C  ->  (X + (Y << C)) & (~0 << C)
        if (isLeftShift && Op0BO->getOperand(0)->hasOneUse() &&
            match(Op0BO->getOperand(0), m_Shr(m_Value(V1),
                  m_Specific(Op1)))) {
          Value *YS =  // (Y << C)
            Builder->CreateShl(Op0BO->getOperand(1), Op1, Op0BO->getName());
          // (X + (Y << C))
          Value *X = Builder->CreateBinOp(Op0BO->getOpcode(), V1, YS,
                                          Op0BO->getOperand(0)->getName());
          uint32_t Op1Val = COp1->getLimitedValue(TypeBits);

          APInt Bits = APInt::getHighBitsSet(TypeBits, TypeBits - Op1Val);
          Constant *Mask = ConstantInt::get(I.getContext(), Bits);
          if (VectorType *VT = dyn_cast<VectorType>(X->getType()))
            Mask = ConstantVector::getSplat(VT->getNumElements(), Mask);
          return BinaryOperator::CreateAnd(X, Mask);
        }

        // Turn (((X >> C)&CC) + Y) << C  ->  (X + (Y << C)) & (CC << C)
        if (isLeftShift && Op0BO->getOperand(0)->hasOneUse() &&
            match(Op0BO->getOperand(0),
                  m_And(m_OneUse(m_Shr(m_Value(V1), m_Value(V2))),
                        m_ConstantInt(CC))) && V2 == Op1) {
          Value *YS = // (Y << C)
            Builder->CreateShl(Op0BO->getOperand(1), Op1, Op0BO->getName());
          // X & (CC << C)
          Value *XM = Builder->CreateAnd(V1, ConstantExpr::getShl(CC, Op1),
                                         V1->getName()+".mask");

          return BinaryOperator::Create(Op0BO->getOpcode(), XM, YS);
        }

        break;
      }
      }


      // If the operand is a bitwise operator with a constant RHS, and the
      // shift is the only use, we can pull it out of the shift.
      if (ConstantInt *Op0C = dyn_cast<ConstantInt>(Op0BO->getOperand(1))) {
        bool isValid = true;     // Valid only for And, Or, Xor
        bool highBitSet = false; // Transform if high bit of constant set?

        switch (Op0BO->getOpcode()) {
        default: isValid = false; break;   // Do not perform transform!
        case Instruction::Add:
          isValid = isLeftShift;
          break;
        case Instruction::Or:
        case Instruction::Xor:
          highBitSet = false;
          break;
        case Instruction::And:
          highBitSet = true;
          break;
        }

        // If this is a signed shift right, and the high bit is modified
        // by the logical operation, do not perform the transformation.
        // The highBitSet boolean indicates the value of the high bit of
        // the constant which would cause it to be modified for this
        // operation.
        //
        if (isValid && I.getOpcode() == Instruction::AShr)
          isValid = Op0C->getValue()[TypeBits-1] == highBitSet;

        if (isValid) {
          Constant *NewRHS = ConstantExpr::get(I.getOpcode(), Op0C, Op1);

          Value *NewShift =
            Builder->CreateBinOp(I.getOpcode(), Op0BO->getOperand(0), Op1);
          NewShift->takeName(Op0BO);

          return BinaryOperator::Create(Op0BO->getOpcode(), NewShift,
                                        NewRHS);
        }
      }
    }
  }

  if (Instruction *Folded = foldShiftByConstOfShiftByConst(I, COp1, Builder))
    return Folded;

  return nullptr;
}

Instruction *InstCombiner::visitShl(BinaryOperator &I) {
  if (Value *V = SimplifyVectorOp(I))
    return replaceInstUsesWith(I, V);

  Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
  if (Value *V = SimplifyShlInst(Op0, Op1, I.hasNoSignedWrap(),
                                 I.hasNoUnsignedWrap(), DL, &TLI, &DT, &AC))
    return replaceInstUsesWith(I, V);

  if (Instruction *V = commonShiftTransforms(I))
    return V;

  const APInt *ShAmtAPInt;
  if (match(Op1, m_APInt(ShAmtAPInt))) {
    unsigned ShAmt = ShAmtAPInt->getZExtValue();

    // shl (zext X), ShAmt --> zext (shl X, ShAmt)
    // This is only valid if X would have zeros shifted out.
    Value *X;
    if (match(Op0, m_ZExt(m_Value(X)))) {
      unsigned SrcWidth = X->getType()->getScalarSizeInBits();
      if (ShAmt < SrcWidth &&
          MaskedValueIsZero(X, APInt::getHighBitsSet(SrcWidth, ShAmt), 0, &I))
        return new ZExtInst(Builder->CreateShl(X, ShAmt), I.getType());
    }

    // If the shifted-out value is known-zero, then this is a NUW shift.
    if (!I.hasNoUnsignedWrap() &&
        MaskedValueIsZero(
            Op0, APInt::getHighBitsSet(ShAmtAPInt->getBitWidth(), ShAmt), 0,
            &I)) {
      I.setHasNoUnsignedWrap();
      return &I;
    }

    // If the shifted-out value is all signbits, then this is a NSW shift.
    if (!I.hasNoSignedWrap() && ComputeNumSignBits(Op0, 0, &I) > ShAmt) {
      I.setHasNoSignedWrap();
      return &I;
    }
  }

  // (C1 << A) << C2 -> (C1 << C2) << A
  Constant *C1, *C2;
  Value *A;
  if (match(Op0, m_OneUse(m_Shl(m_Constant(C1), m_Value(A)))) &&
      match(Op1, m_Constant(C2)))
    return BinaryOperator::CreateShl(ConstantExpr::getShl(C1, C2), A);

  return nullptr;
}

Instruction *InstCombiner::visitLShr(BinaryOperator &I) {
  if (Value *V = SimplifyVectorOp(I))
    return replaceInstUsesWith(I, V);

  Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
  if (Value *V = SimplifyLShrInst(Op0, Op1, I.isExact(), DL, &TLI, &DT, &AC))
    return replaceInstUsesWith(I, V);

  if (Instruction *R = commonShiftTransforms(I))
    return R;

  const APInt *ShAmtAPInt;
  if (match(Op1, m_APInt(ShAmtAPInt))) {
    unsigned ShAmt = ShAmtAPInt->getZExtValue();
    unsigned BitWidth = Op0->getType()->getScalarSizeInBits();
    auto *II = dyn_cast<IntrinsicInst>(Op0);
    if (II && isPowerOf2_32(BitWidth) && Log2_32(BitWidth) == ShAmt &&
        (II->getIntrinsicID() == Intrinsic::ctlz ||
         II->getIntrinsicID() == Intrinsic::cttz ||
         II->getIntrinsicID() == Intrinsic::ctpop)) {
      // ctlz.i32(x)>>5  --> zext(x == 0)
      // cttz.i32(x)>>5  --> zext(x == 0)
      // ctpop.i32(x)>>5 --> zext(x == -1)
      bool IsPop = II->getIntrinsicID() == Intrinsic::ctpop;
      Constant *RHS = ConstantInt::getSigned(Op0->getType(), IsPop ? -1 : 0);
      Value *Cmp = Builder->CreateICmpEQ(II->getArgOperand(0), RHS);
      return new ZExtInst(Cmp, II->getType());
    }

    // If the shifted-out value is known-zero, then this is an exact shift.
    if (!I.isExact() &&
        MaskedValueIsZero(Op0, APInt::getLowBitsSet(BitWidth, ShAmt), 0, &I)) {
      I.setIsExact();
      return &I;
    }
  }
  return nullptr;
}

Instruction *InstCombiner::visitAShr(BinaryOperator &I) {
  if (Value *V = SimplifyVectorOp(I))
    return replaceInstUsesWith(I, V);

  Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
  if (Value *V = SimplifyAShrInst(Op0, Op1, I.isExact(), DL, &TLI, &DT, &AC))
    return replaceInstUsesWith(I, V);

  if (Instruction *R = commonShiftTransforms(I))
    return R;

  unsigned BitWidth = I.getType()->getScalarSizeInBits();
  if (ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
    unsigned ShAmt = Op1C->getZExtValue();

    // If the shift amount equals the difference in width of the destination
    // and source types:
    // ashr (shl (zext X), C), C --> sext X
    Value *X;
    if (match(Op0, m_Shl(m_ZExt(m_Value(X)), m_Specific(Op1))) &&
        ShAmt == BitWidth - X->getType()->getScalarSizeInBits())
      return new SExtInst(X, I.getType());

    // If the shifted-out value is known-zero, then this is an exact shift.
    if (!I.isExact() &&
        MaskedValueIsZero(Op0, APInt::getLowBitsSet(BitWidth, ShAmt), 0, &I)) {
      I.setIsExact();
      return &I;
    }
  }

  // See if we can turn a signed shr into an unsigned shr.
  if (MaskedValueIsZero(Op0, APInt::getSignBit(BitWidth), 0, &I))
    return BinaryOperator::CreateLShr(Op0, Op1);

  return nullptr;
}